1. Field of the Invention
The present invention relates generally to an improved base for a roadway marker and more particularly to a marker base having improved pavement adherence capabilities when used with conventional adhesives.
2. Description of the Prior Art
Roadway markers have long been used to designate lanes of traffic and roadway centerlines. Early forms of roadway markers were constructed of a solid piece of ceramic material having a high gloss. Bumped protrusions were formed on the bottom surfaces of these markers to increase surface area for cooperation with a suitable adhesive material.
In more recent times, roadway markers evolved to the use of thermoplastics to form marker shells. For purposes of manufacturing economies, these markers are not formed into a solid structures like the early ceramic markers. Rather, they typically have a relatively thin wall with a series of internal ribs defining a plurality of chambers, much like in a waffle or honeycomb configuration. An example of such a marker is disclosed in U.S. Pat. No. D-267,933. In order to mold these markers, the ribs or chamber walls are downwardly tapered to a narrow dimension at the roadway surface. This is necessary to provide sufficient draft angle for removal of the marker from the mold. These markers are installed by depositing a suitable adhesive such as bitumen or epoxy on the roadway surface and pressing the marker downwardly into the adhesive.
It has been found that a disadvantage of the foregoing markers is that when the marker is pressed into the adhesive it exhibits a "cookie cutter" effect. Essentially, the narrow bottom surfaces of the chamber walls squeeze the adhesive upwardly into the chambers leaving little adhesive between the bottom marker surfaces and the roadway to anchor the marker firmly.
In order to improve the adherence capabilities of markers, a marker is known wherein the ribs or internal chamber walls are initially formed with the aforementioned tapering configuration. Then, in a separate manufacturing step, the bottom surfaces of the walls are thermally deformed to have a mushroom-like cross-section. This deformation essentially increases the bottom area of the walls and reduces the cookie cutter effect of the marker. The mushrooming of the walls also allows adhesive to flow into the chambers and over the tops of mushroom surfaces thereby providing an enhanced mechanical bond between the marker and the adhesive. An example of such a marker is disclosed in U.S. Pat. No. 5,078,538 issued Jan. 7, 1992 and assigned to the common assignee herein.
While markers as just described improve over earlier constructions, they are still not without disadvantages. Specifically, it has been found that markers having internal chambers sealed at their bottoms with adhesive experience pressure differentials between the ambient air pressure acting on the exposed shell of the marker and the air pressure internal to the chambers. Air pressure within the marker chambers increases notably, for example, if the marker is installed cold and then later is heated by the elevated ambient temperatures. This effect of differential pressure reduces the hold-down force on the marker. Another problem experienced by conventional markers is that a preferred adhesive material for installing markers is bitumen. Bitumen can be readily heated to a fluid state and poured onto the pavement in puddles at locations where markers are desired. The markers may then be pressed into the puddles and the bitumen may be allowed to cool. The bitumen hardens quickly as compared to alternative adhesives such as epoxy. A disadvantage of bitumen, however, is that it dramatically loses adhesive bonding strength with temperature. Thus, even with improved markers of the type disclosed in the aforementioned U.S. Pat. No. 5,078,538, the surface area of the typical marker bottom is still not sufficient to create a strong enough bond between the marker and bituminous adhesive, particularly at elevated temperatures.
Accordingly, it is desirable to provide an improved marker having a honeycomb interior for strength, but which does not exhibit a cookie cutter effect. It is further desirable to provide such a marker which is resistant to dislodging under the effects of air pressure differentials between ambient pressure and internal marker pressure. Still further, it is desirable to provide such a marker with a base having a relatively large surface area for enhanced mechanical bonding with preferred adhesive materials such as bitumen.